Bipolar transistors typically are formed in single crystal silicon. Single crystals of other semiconductor materials, such as germanium, silicon germanium and other semiconductor materials, also are sometimes used. Polycrystalline forms of silicon (“polysilicon”), germanium (“polygermanium”), and silicon germanium (“polysilicon polygermanium”) typically are not used to make bipolar transistors because usually these materials do not provide sufficient current gain for electronic applications of this type. This is because the current gain of a bipolar transistor is related to the lifetime of minority carriers in its base. In typical single-crystal silicon substrates, for example, these lifetimes can be in excess of 10−3 sec. However, for typical polysilicon films, these lifetimes are on the order of 10−12 sec. Carrier lifetimes are determined by the density of defects in the material, which act as recombination centers that take free carriers out of the film. The polysilicon films have very high defect densities, which accounts for the low carrier lifetimes.
One application for a transistor is as a memory select device for a three dimensional memory array of one time programmable or rewriteable memory cells. The array typically extends above a substrate, which typically is a silicon wafer or die, but also may include other materials, such as, for example, glass, other semiconductor materials, metal, plastic, silicon dioxide or aluminum oxide. These memory select devices are usually disposed on the substrate, not up in the array of memory cells.
One type of three dimensional memory array is a stacked, monolithic three dimensional memory array. The term “monolithic” as used herein means that layers of each level of the array were directly deposited or grown on the layers of each underlying level of the array. In contrast, two dimensional arrays may be formed separately and then packaged together to form a non-monolithic memory device. In general, to program a given memory cell within the three dimensional memory array, it is necessary to have multiple memory select devices, which direct a programming voltage to the particular cell that is to be programmed in response to signals from control circuitry disposed on the substrate. For monolithic three dimensional memory arrays in particular, memory select devices, such as bit-line select transistors, typically can take up anywhere from around 10-20% of the total die area underneath the array. This leaves less space for control and other circuitry in the substrate or requires a larger substrate base to incorporate the control circuitry and the memory select transistors.